The present invention relates to a device for determining a (mass) flux of a fluid and a (mass) flux of a substance dissolved in the fluid. In particular, the fluid is a liquid, and the dissolved substance is a pollutant or a mineral. Devices in accordance with the invention are particularly suitable for measuring in underground liquid layers, such as groundwater layers.
Groundwater may be polluted by a large number of various pollutants, ranging from organic and inorganic substances, such as volatile organic hydrocarbons, mineral oil, phosphates, nitrates and pesticides, to heavy metals. For ecological and health reasons, there is growing interest in efficient risk management of groundwater which, in the case of (residual) pollution, is often the only and best available technology not incurring excessive costs. To this end, the groundwater pollution and/or groundwater fluxes have to be efficiently and accurately mapped. Only then will it be possible to correctly assess the ecological and health impact of the pollution and take any necessary action to counteract groundwater pollution and/or to limit the effects thereof.
In order to determine the quality of the groundwater, it is known to drill monitoring wells as far as the groundwater layer and to regularly take a groundwater sample via these monitoring wells using a pump and analyse it. This system only produces snapshots. As it is known that the concentration of a pollutant in the groundwater may fluctuate greatly, there is a risk that a distorted image of the groundwater pollution is obtained.
U.S. Pat. No. 5,996,423 and US 2014/0290391 describe devices for taking samples from groundwater, which are based on diffusion. These devices comprise a housing which is filled with distilled water. The housing is surrounded by a semi-permeable membrane, which is permeable to the substance to be sampled which is dissolved in the groundwater and non-permeable to the groundwater itself. The substance to be sampled will diffuse to the distilled water until an equilibrium is reached. Such systems are able to determine a time-averaged concentration of a pollutant. However, the concentration value on its own does not suffice to determine the amount of pollutant which spreads via the groundwater.
WO 01/33173 and U.S. Pat. No. 7,325,443 describe devices for determining mass transport of groundwater and of a substance dissolved in the groundwater. These devices comprise a porous matrix through which liquids can flow and which contains sorbents for the dissolved substance which is to be measured. In addition, these devices comprise so-called tracers by means of which the groundwater flux can be determined. The tracers are either impregnated in the porous matrix (WO 01/33173), or mixed therewith, or arranged separately in such a manner that they can diffuse into the porous matrix (U.S. Pat. No. 7,325,443). A drawback of such arrangements is the fact that the laboratory analysis is difficult, since mutual interference has to be taken into account. After all, the adsorption of certain pollutant molecules onto the porous matrix may affect the diffusion of the tracers and vice versa. In addition, diffusion and non-equilibrium conditions may result in a distorted image of the tracer movement.